When scientists talk about the cryosphere, they mean the places on Earth where water is in its solid form, frozen into ice or snow. Read more ...

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Arctic sea ice before satellites

Last week, a reader of Arctic Sea Ice News & Analysis asked what we know about Arctic sea ice extent before the satellite records began in 1979. Those records show that Arctic sea ice has been declining at an increasing pace since 1979—enough data to see a strong signal of climate change. But scientists also want to know what sea ice was like before satellites were there to observe it. Mark Serreze, NSIDC director and research scientist, said, “The better we understand how the climate system behaved in the past, the better we can understand and place into context what is happening today.” What do we know about sea ice conditions before 1979, and how do we know that?

Sea ice charts of the Arctic Ocean show that ice extent has declined since at least the 1950s. Credit: NSIDC and the UK Hadley Center

Historical data on sea ice

Scientists have pieced together historical ice conditions to determine that Arctic sea ice could have been much lower in summer as recently as 5,500 years ago. Before then, scientists think it possible that Arctic sea ice cover melted completely during summers about 125,000 years ago, during a warm period between ice ages.

To look back into the past, researchers combine data and records from indirect sources known as proxy records. Researchers delved into shipping charts going back to the 1950s, which noted sea ice conditions. The data gleaned from those records, called the Hadley data set, show that Arctic sea ice has declined since at least the mid-1950s. Shipping records exist back to the 1700s, but do not provide complete coverage of the Arctic Ocean. However, taken together these records indicate that the current decline is unprecedented in the last several hundred years.

Researchers use historical shipping records as one source of sea ice data. The HMS Investigator sank after getting caught in thick ice in the Northwest Passage in 1853. Credit: Toronto National Reference Library

Before the 1950s, the data are patchier. So researchers also use clues from the environment to look into past sea ice conditions. Core samples from the ocean floor allow scientists to study layers of marine sediments laid down hundreds, thousands, even millions of years ago. They have also studied the remains of algae, plants, and animals in the ocean floor or along coastlines. Ice cores pulled from deep within Arctic glaciers contain evidence of past temperatures and periods of cooling and warming. Even the distribution of ancient driftwood can provide clues about where there was open water and where there was ice. While a single piece of evidence does not provide a whole picture of past conditions, many of them together can add up to a more complete picture of historical ice conditions.

The historical data also show how closely Arctic sea ice extent is linked to Earth’s climate. When Arctic sea ice was lower, Earth’s climate was much warmer than it is today, and sea level was higher than it is today. Leonid Polyak, a researcher at the Byrd Polar Research Center, said, “If that’s where we are heading, we should be worried.”